The present invention relates to a catalyst for exhaust purification, which can purify exhaust from a combustion engine such as a vehicle or a diesel car. And particularly it relates to an electrochemical catalyst for exhaust purification, which can store both NOx and hydrocarbons and electrochemically react NOx, with hydrocarbons.
A conventional catalyst for exhaust purification, as typically a ternary catalyst, has played an important role in reducing an exhausted quantity of NOx, CO and HC by carrying out a reduction reaction reducing nitrogen oxides such as NO and N02 to nitrogen and an oxidation reaction oxidizing reductive substances such as hydrocarbons (HC) and CO on the same catalyst. However, methods for burning fuel under a lean condition such as a lean-burn engine and GDI (gas direct injection) engine are used in order to improve efficiency of a combustion engine, making it difficult to constantly supply HC and CO in an amount for sufficiently reducing NOx.
For this reason, there is used a method, in which NOx is temporarily stored by using an NOx absorbing substance, such as barium carbonate (BaCO3) and, a rich driving mode is occasionally carried out in every one or two minutes supplying HC and CO to reduce the stored NOx. However, since the NOx absorbing substance deteriorates due to sulfur poisoning and NOx absorbing capability is changed, it is difficult to predetermine timing for providing the rich driving mode and timing for a reactivation process for the sulfur poisoning. And it is necessary to drive while always monitoring using an air-ratio sensor, an oxygen sensor or the like. Moreover, there arises a problem such as torque shock upon inserting a rich spike, depending on driving modes.
HC, in addition to NOx, can be temporarily stored by using an HC adsorption substance together with an NOx absorption substance. For example, Japanese Unexamined Patent Publication NO. 10-225636/1998 discloses a method for placing an HC adsorption substance around the NOx absorbing substance. FIG. 11 is an explanatory drawing of a catalyst for exhaust purification disclosed in the abovementioned publication. In the FIG., 30 represents a particle of platinum catalyst, 31 represents a particle of rhodium catalyst, 32 represents a first powder (mordenite type zeolite) functioning as a support, 33 represents second powder (Ba/alumina) containing Ba as an NOx absorption substance, and 34 represents zirconia as an adsorption substance for HC. In a rich atmosphere, HC is adsorbed by zirconia 34, and hydrogen is generated on the Rh catalyst particle 31 by a reaction of HC and water vapor, and the hydrogen reduces NOx. Moreover, in a lean atmosphere, NOx is absorbed by barium carbonate contained in the second powder 33. When, the atmosphere is temporarily changed to a stoichiometric ratio (stoichiometric air-fuel ratio) or a rich atmosphere, NOx absorbed by the second powder 33 as the NOx absorption substance is released, so that NOx is reduced to N2 by reacting with HC and CO in exhaust according to the catalytic function of Pt 30 and Rh 31. Therefore, zirconia 34 as an HC adsorption substance is used to generate hydrogen by reaction with water vapor, and its purpose is to generate hydrogen as a reduction substance having high reactivity. Namely, even if zirconia 34 as an HC adsorption substance is placed around the second powder 33 as an NOx absorption substance, it is impossible to directly react NOx absorbed in the second powder 33 as an NOx absorbing substance with HC adsorbed in zirconia 34. This is because the reduction substance and the oxidation substance can not be reacted with each other, unless they are on the same catalyst particle, whether it is Pt or Rh. Namely, unless the oxidation substance and the reduction substance are placed together in a gaseous phase, it is impossible to reduce the oxidation substance (converting NOx to nitrogen) and to oxidize the reduction substance (converting HC to carbon dioxide and water). This fact has been commonly recognized in a long history of the conventional catalyst for exhaust, and it is inevitable to carry out both oxidation and reduction through chemical reactions on a catalyst. Therefore, even if zirconia 34 as an adsorption substance for HC is placed around the second powder 33 serving as an NOx absorbing substance, it is impossible to reduce NOx unless a rich spike process is carried out.
With respect to effects obtained when an adsorption substance for HC is placed around an NOx absorbing substance, Japanese Unexamined Patent Publication NO. 10-57763/1998 describes that the installation of the HC adsorption substance increases the conversion to N2, although not less than half of NOx is not converted to N2, but to N2O by using only the NOx absorbing substance.
Besides these, as a mixed catalyst for exhaust purification, there is a catalyst obtained by merely mixing an oxidizing catalyst and a reducing catalyst, which is disclosed in Japanese Unexamined Patent Publication NO. 1-139144/1989. However, since an oxidation reaction and a reduction reaction are independent chemical reactions, the effect of accelerating the oxidation reaction and the reduction reaction is small, and it is necessary that the reducing reagent and the oxidizing reagent exist in a sufficient amount in a gaseous phase.
Moreover, as a method for electrochemically reducing NOx, Japanese Unexamined Patent Publication NO. 4-305227/1992 discloses a device in which a gas containing NOx is supplied to a cathode side and a gas mixture containing hydrogen and CO is supplied to an anode side so that electric power is generated at 650° C. by using a fused carbonate as an electrolyte, and an electromotive force of at least 800 mV is obtained as a releasing voltage. However, this system has been invented as a combustion exhaust processing method for a power plant, and it can not be applied to an automobile from the viewpoint of volume, weight, control, costs and the like.
Furthermore, as a method for electrochemically reducing NOx, Japanese Unexamined Patent Publication NO. 7-275714/1995 discloses a device in which a gas containing NOx is supplied to a cathode side, and a voltage is applied between cathode and anode at 500 to 700° C. by using a solid electrolyte of zirconia stabilized with yttria as an electrolyte, so that an electrolytic process is carried out to reduce NOx, thereby releasing oxygen from the cathode. And Japanese Unexamined Patent Publication NO. 8-332342/1998 discloses a device in which an electrolytic process is carried out at 400 to 1000° C. by using perovskite ceramics as an ion conductor conductive and an electron conductive material so that NOx is reduced, thereby releasing oxygen from the cathode. However, these systems have also been devised as a combustion exhaust processing method for a power plant, and have not been applied to an automobile from the viewpoint of volume, weight, control, costs, and the like.
As described above, in the conventional catalyst for exhaust purification, an oxidation reaction and a reduction reaction are simultaneously carried out on the same catalyst particle, so NOx can not be reduced unless a reducing reagent such as HC, CO or hydrogen exists in a sufficient concentration in a gaseous phase. Therefore, in case of driving at a lean atmosphere, it is necessary to temporarily switch to a rich atmosphere to reduce NOx absorbed in an NOx absorbing substance.
The present invention has been carried out to solve the above-mentioned conventional problems, and its object is to provide a catalyst for exhaust purification which can sufficiently function even at a lean atmosphere by electrochemically carrying out an oxidation reaction and a reduction reaction, not by simply carrying out a chemical catalytic reaction as in the conventional catalyst for exhaust purification.
Additionally, an “electrochemical catalyst” for electrochemically carrying out an oxidation reaction and a reduction reaction by using at least two kinds of catalyst was disclosed in Japanese Unexamined Patent Publication NO. 10-270055/1998, which was previously filed by the inventors of the present invention. Moreover, a method for improving poisoning resistance to CO of a fuel battery by using the “electrochemical catalyst” was disclosed in Japanese Unexamined Patent Publication NO. 10-270056/1998, which was previously filed by the inventors of the present invention. The present invention has been made by basically applying the above-mentioned “electrochemical catalyst” for an exhaust purification catalyst, and the effect of the “electrochemical catalyst” under a special environment of the exhaust is improved by providing an NOx absorbing substance and a hydrocarbon adsorption substance.